This invention relates to the control of the transmission of digital information which is composed of data blocks separated by gaps.
When digital information is recorded in blocks a synchronising system is normally used, on playback of the information, to establish the identity of the data words within the blocks of data. For example, in a recorder of audio information in digital form, the data bits within a block are, before they are recorded on the magnetic tape, usually rearranged in time sequence, in order to facilitate the detection and correction of error. During playback, the data must be rearranged in order to recover the data words in their original forms and order. The rearrangement is normally performed with the aid of a buffer memory. The process of writing information into the buffer memory must be exactly synchronised else otherwise the data may be written incorrectly in the various locations of the memory and the information will not be reassembled in its proper order. Other systems for the transmission of data have, for similar reasons, a similar requirement for the correct synchronisation of the data.
It has been proposed to provide, at the beginning of each data block, a control word so that the detection of this control word may provide a synchronising signal. However, many systems and machines, particularly tape recorders, for the recording or playback of digital information suffer from "drop-outs", that is to say temporary losses of information, owing to, for example, imperfections in the tape or other storage medium. The loss of all or part of a synchronising control word cannot be tolerated in any system which requires active synchronisation.